1. Field of the Invention
The present invention relates to a digital image processing technique, and more particularly, to a histogram stretching apparatus and a histogram stretching method which can enhance contrast of a digital image by analyzing a histogram of the image and stretching the histogram, so that the image sharpness can be improved.
2. Description of the Related Art
The contrast of an image means the degree of difference between dark and bright parts of the image. As the difference becomes greater, the contrast is enhanced. A high-contrast image may be considered as an image having high sharpness due to the clear difference between dark and bright parts of the image. If an image having low contrast is processed and converted into an image having high contrast, the sharpness of the corresponding image is improved and thus a high-quality image can be obtained.
Image histogram stretching is well known as one of image processing techniques for enhancement of the sharpness of a low-contrast image. A histogram of a high-contrast image has a wide frequency distribution over entire image levels. That is, the high-contrast image has sufficient frequencies in both dark and bright parts of the image and the frequencies of the high-contrast image are widely distributed over the entire image levels in the histogram. In contrast, a histogram of a low-contrast image has a relatively narrow frequency distribution with respect to image levels because the low-contrast image has a small difference between dark and bright parts of the image. In this case, an image having a clear difference between dark and bright parts thereof can be acquired by expanding the narrow frequency distribution of the low-contrast image. Such expansion of the frequency distribution of the histogram is called histogram stretching.
The image histogram stretching technique can contribute to enhancement of image sharpness if it is applied to an image acquired under normal circumstances. However, if the histogram stretching technique is applied to an image acquired under some special conditions, image distortion may occur, lowering the image quality. For example, the image enhancement may not be achieved or the image distortion may occur if the histogram stretching is collectively applied to an image having a very bright light source, i.e., a saturated portion therein, a monochromatic image having a very narrow histogram distribution, and an image having a histogram distribution skewed toward a high image levels (i.e., a bright side) or low image levels (i.e., a dark side).
Accordingly, there is a need for a technique allowing the histogram stretching to be performed properly in due consideration of capturing conditions, i.e., histogram distribution states of an input image on which the histogram stretching is to be performed.